The secondary phloem of Austrobaileya scandens

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Abstract
The origin of sieve elements and parenchyma cells in the secondary phloem of Austrobaileya was studied by use of serial cross sections stained with tannic acid – ferric chloride and lacmoid. In three important respects, Austrobaileya phloem recalls gymnospermous features: it has sieve cells rather than sieve-tube members; a significant proportion of sieve elements and companion cells arise independently of each other; and sieve areas occur between sieve elements and companion cells ontogenetically unrelated to each other. The angiospermous feature includes origin of most sieve elements and parenchyma, including companion cells, after divisions in phloic initials. In these instances companion cells show a closer ontogenetic relationship to sieve elements than do other parenchyma cells. The combination of gymnospermous and angiospermous features makes phloem of Austrobaileya unique when compared to that of all those species that have been investigated in detail. It is further suggested that the term albuminous cells is inappropriate and should be replaced by companion cells but that the ontogenetic relationship implicit in the definition of companion cells is too restrictive and should be abandoned.

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